This invention relates to the field of sonar, and more particularly to sonar apparatus for the detection and classification of intruders into a water zone of interest.
A variety of approaches have been considered for the detection of the presence or intrusion of things or persons into an air or water zone or area of interest. In general, such approaches have either been passive, listening devices or have been active systems that include transmitter means to insonify the zone of interest, and receiver means that detect characteristic changes or disturbances in the insonification signals. When the transmission and receiving means are located at different points in space but share a common insonification region, the system is termed bistatic.
Bistatic intruder detection devices in general rely on motion induced frequency shifts or Doppler effects. U.S. Pat. Nos. 3,111,657; 3,121,856; 3,383,678; 3,582,870; 3,740,704; 3,744,015; 3,878,526; 3,955,171; and 4,206,510 provide examples of Doppler based detection systems. Among the advances taught by those patents are the use of automatic gain control to adjust receiver gain to background noise conditions, the use of plural filters in detection of frequency related returns, the averaging of returns over a period of time, and use of spectral energy content and slope against selected threshhold values. Bistatic sonar detection of intrusion into a building, room, or other air space differs from detection of an intruder in a water zone of concern in numerous respects including but not limited to differences in speed of sound transmission, reverberation, background noise, type and speed of body movements relative to the background, and power considerations. It has been determined, for example, that the strokes of a swimmer or of a wader, or a small boat paddler, produce Doppler variations concentrated at the higher end of the range of frequencies expected. This fact is taken advantage of by the invention to achieve a more certain logic classification and lower false alarm rate.
Among the shortcomings of existing systems for detection of an intruder into a water zone of interest are low assurance of detection of certain intruder characteristics, high false alarm rate, uneven area or volume coverage, high installation and maintenance costs, and a need for skilled and attentive operators.